1. Field of the Invention
The invention relates to new anisotropic compounds with negative or positive dielectric constant anisotropy and low, e.g., 0.2 optical anisotropy (.DELTA.n) at the most, as well as liquid crystal (LC) compositions containing these anisotropic compounds and are used as the dielectric for liquid crystal indicators.
2. Description of the Prior Art
The anisotropic compounds known up till now for liquid crystal compositions whose dielectric constant-anisotropy, abbreviated DCA or .DELTA..epsilon., is negative, are mostly of two or three combinations, made up of aromatic rings with one of two carboxyl groups and, if necessary, a covalent bond as a bridge between the rings, and with one or more side groups, i.e., core substituent groups not lying in the molecular longitudinal axis, such as nitrile or nitro groups, particularly, such as described in German AS No. 2 240 864, German OS Nos. 2 613 293, 2 835 662 and 2 836 086. These known anisotropic compounds with negative DCA (.DELTA..epsilon.), however, have high viscosity values as well as high values for optical anisotropy (.DELTA.n) (.DELTA..epsilon.=.epsilon..sub.11 -.epsilon..sub..perp. and .DELTA.n=n.sub.11 -n.sub..perp.). The DCA of a compound is negative if the dielectric constant (.epsilon.) parallel to the molecular axis is smaller than the dielectric constant perpendicular to the molecular axis. The optical anisotropy (.DELTA.n) of a compound is positive if its refraction index (n.sub.195) perpendicular to the molecular axis is smaller than its refraction index (n.sub.11) parallel to the molecular axis; this optical anisotropy value, i.e., .DELTA.n=n.sub.11 -n.sub..perp. is designated "low" here if it is smaller than, or at the most 0.2 (.DELTA.n.ltoreq.0.2). Suitable methods for measuring .DELTA.n are known in the literature (e.g., I. Haller et al, Mol. Cryst. and Liqu. Cryst. 16/1972/53-59) and usually refer to a definite light wave length (here 633 nm).
It is known from the literature, e.g., German OS No. 2 636 684, that the viscosity of liquid crystalline and linear aromatic compounds with no side groups, such as biphenyl nitriles, can be reduced by substituting a cyclohexane ring for one of the phenylene rings. This possibility for producing anisotropic compounds with high negative DC-anisotropy and low .DELTA.n-values suited for LC compositions has remained unused until now. To be sure such compounds with a side group, e.g., methyl or ethyl groups or chlorine atoms, were mentioned in the German OS No. 2 752 975 among a number of specially described, linear, tri-cyclic diesters with one or two benzol-ring(s) and two or one cyclohexane ring(s); although mention was made of the possibility of a side nitrile group, it was not recognized that a new group of advantageous anisotropic compounds with negative, not to mention strongly negative, DC-anisotropy would have been made accessible according to the structural principle disclosed there.
Anisotropy, i.e., enantiotropic liquid crystalline or nonotropic or potentially liquid crystalline compounds with high negative DC-anisotropy and low .DELTA.n-values are required for LC-indicators, as was described by the applicant in the German PA No. 35 863.6, which utilize the so-called "inverse Guest-Host-effect"; furthermore, anisotropic compounds with such characteristics are also required for certain types of dynamic (scatter/leakage) cells. However, prior known anisotropic compounds with negative DC anisotropy do not satisfy the required conditions or at least not to a satisfactory degree.
A need therefore continues to exist for new anisotropic compounds with negative DCA and low optical anisotropy (.DELTA.n), e.g., for LC-indicators with multiplex operation.